The manual says "When making unbalanced connections, the unused terminal must be grounded for proper response." So, regular mono 1/4" plugs should do that for you, but the signal will be inverted. If polarity matters, use 1/4" stereo plugs, make the ring "hot" and ground the tip to the shell. So, the simplest solution is a 3.5mm to RCA cable, then two mono RCA to 1/4" cables.